Recently, more and more attention is paid on passive building design strategies, including night ventilation. In the present paper, a numerical study is presented to investigate unsteady flow and heat transfer around and through an isolated high-rise building (with a 1:1:2 shape) based on night ventilation and thermal mass. With the aid of computational fluid dynamics (CFD), a three-dimensional unsteady mathematical model is established to describe wind flow around and through an isolated high-rise building. Inside the building, the Brinkman-Forchheimer extended Darcy model and the local thermal non-equilibrium (LTNE) model are employed for the first time, to describe flow and heat transfer between air ventilation and thermal mass. The renormalization group theory (RNG) k-ε model and the scalable wall function are employed in the turbulence modeling. The reliability of the mathematical model is validated with published wind tunnel experimental data as well as simulation results. After that, flow and heat transfer characteristic, cooling effects of night ventilation are obtained. The effects of three key parameters: airflow velocity, airflow temperature and porosities of the building are investigated in detail. The whole numerical analysis contributes towards a better understanding and evaluation of the night ventilation.